The present invention relates to homo- or copolymers of 1,3-dienes carrying reactive silyl groups, to a process for their production, and their use as adhesion promoters in the preparation of mixtures of polymers, mineral fillers and, optionally, other additives.
It is known to use carbon blacks of various specifications as components of elastomer mixtures. It is furthermore known that one of the purposes of the addition of carbon black is to make resultant vulcanizates less expensive but more importantly to raise the overall level of the useful properties. Among these, most important are tear strength, modules, hardness, tear propagation resistance, and abrasion resistance. Therefore, carbon black is classified as a so-called active or reinforcing filler.
However, for various reasons, the use of carbon black in elastomer mixtures is limited. On the one hand, it is only possible to prepare black mixtures with it; in any event, no colored or white mixtures. On the other hand, high-quality carbon blacks have become so expensive as compared with the cheap mineral fillers, such as silicic acid (SiO.sub.2), kaolin, aluminum hydroxide, and glass, that increasing efforts have been made to replace carbon blacks by the light-colored fillers, such as those indicated above. This substitution also reduces the proportion of components based on petroleum, which is subject to supply crises. Moreover, the use of highly active silicic acids is of great advantage for optimizing specific properties, e.g. notch toughness.
Such light-colored mineral fillers have been utilized heretofore, but primarily form the point of view of merely rendering the product less expensive. Initially, this meant considerable losses in properties valuable from the viewpoint of the technology of particular end uses, such as, for example, heat degradation, elasticity, and compression set. Similar problems also exist in filling and/or reinforcing other polymeric materials with mineral fillers, for example polyolefins or unsaturated polyester resins.
It is known that these disadvantages can be eliminated, at least in part, by the use of so-called adhesion promoters. In general, there are materials having a certain affinity to the polymer as well as the filler, expressed preferably in the capability of entering into a chemical reaction with the two substrates.
The organofunctional silanes have become especially well known as adhesive promoters. They have the general formula R-SiX.sub.3 wherein X in most cases is alkoxy and less frequently halogen, and the organic residue R is alkyl or aryl substituted by a functional group. These compounds do yield satisfactory results with respect to the properties of the resultant polymer-filler combinations, but they have several disadvantages when used. Thus, in vulcanizable elastomer-filler mixtures various silanes are each suitable optimally only for a specific type of crosslinking technique. Also, unpleasant odors are encountered, for example, when using the mercaptosilanes. Moreover, there is a tendency toward premature vulcanization of the mixtures combined with such compounds (scorching). Moreover, organofunctional silanes, as compared to the other components of the elastomer mixture, are extraordinarily expensive and, in general, exhibit a toxicity by inhalation and skin contact which cannot be neglected.
Furthermore, a great number of attempts has been made to synthesize such adhesion promoters having similar effects and on a polymeric basis. Thus, it has been known, for example, that natural rubber and styrene-butadiene elastomer (SBR) can be hydrosilylated (reacted with hydrosilane) by heating with trichlorosilane to about 300.degree. (U.S. Pat. No. 2,475,122) and that such reaction products adhere well to glass plates (U.S. Pat. No. 2,557,778).
The photochemically based hydrosilylation of a liquid polybutadiene obtained by anionic polymerization is described in U.S. Pat. No. 2,952,576, relating to glass fibers coated with this material for the reinforcement of unsaturated polyester resins. The microstructure of the liquid polybutadiene employed is not mentioned; however, from the data on its manufacture by means of a sodium suspension, in comparison with literature disclosures, it can be concluded that it contains about 60-70% vinyl groups, as well as 30-20% trans-vinylene groups, and only about 10% cis-vinylene groups.
The catalysis of hydrosilylation of polybutadienes by platinum compounds is described in DOS's [German Unexamined Laid-Open Applications] 1,620,934 and 1,720,527 as an intermediate stage in the production of foam stabilizers or laminating resins. These DOS's do not contain any teaching of using the reaction products in connection with elastomer-filler mixtures. Also, as above, both cases involve products having a high vinyl content, whereas the remaining double bonds consist predominantly of trans-vinylene groups. Polybutadienes of this microstructure possess a very high viscosity at room temperature, even at relatively low molecular weights; their handling, metering, and blending are made extraordinarily difficult by this consistency. The same limitations are encountered in the hydrosilylated derivatives thereof.
The conventional platinum catalysis of hydrosilylation is also described in U.S. Pat. No. 3,759,869, wherein polymers are claimed having molecular weights of between 500 and 50,000 and containing, to an extent of at least 25% by weight, the structure ##STR2##
This corresponds, in the case of a pure polybutadiene as the basic polymer, to an attachment of a reactive silyl group -SiX.sub.3 to approximately each tenth monomer unit. The examples reveal only the hydrosilylation of a polybutadiene having an average molecular weight of 1,000 and a vinyl group content of 90%, based on the total number of double bonds, namely with practically 100% saturation of all vinyl groups present. Mixtures of such products, or the derivatives thereof obtained by secondary reactions with low-molecular wieght polypropylene (molecular weight 5,000) and/or with EPM elastomer, are merely mentioned, without any disclosure regarding their effectiveness. Additionally, such most extensively saturated polybutadiene derivatives and/or fillers treated therewith, solely on account of their lack of double bonds, are only poorly suited for forming a composite with a polymer network produced by sulfur or peroxide vulcanization.
DOS, No. 2,343,108 claims the hydrosilylation of elastomer polymers containing at least preferably 5-30% by weight of vinyl groups, and their use as coupling agents for the vulcanization of a vulcanizable elastomer containing a pigment having silicic acid as an ingredient. These are products which, due to their high molecular weight, can only be used in solution.
In contrast thereto, DAS [German Published Application] 2,635,601 describes hydrosilylation products of special polybutadiene oils having molecular weights of 400-6,000, which, thanks to their microstructure (10-60% vinyl groups, 1-15% transvinylene groups, and 25-85% cis-vinylene groups), exhibit particularly low viscosities and therefore can be handled very well in the undiluted form. However, the hydrosilylation products have the disadvantage that the platinum catalyst utilized during their manufacture remains extensively within the product and thus is lost.
Also, the reaction of lithium-terminated "living polymers" with an excess of a tetrahalogeno- or tetraalkoxysilane is known according to the process of U.S. Pat. No. 3,244,664. This excess amount, which must be used to avoid coupling or crosslinking reactions, cannot be practically separated at all and thus is lost to further processing.
Several disclosures are known on the addition of sulfhydryl groups of a mercaptosilane, e.g. of .gamma.-mercaptopropyltriethoxylsilane, to double bonds of an unsaturated polymer (U.S. Pat. No. 3,440,302; DOS Nos. 2,333,566 and 2,333,567), but these processes have the disadvantage of a very expensive and foul-smelling starting material.
Furthermore, processes are known producing polymers with reactive silyl groups using silyl-group-containing peroxy compounds (DOS Nos. 2,152,275 and 2,152,286) or azo compounds (J. Appl. Pol. Sci. 18: 3259 [1974]) as the initiators, or using silyl-group-containing disulfides (DOS No. 2,142,596) as chain-transfer agents in radical polymerizations. Here again, the auxiliary agents utilized for introduction of the silyl groups are hard to obtain, very expensive, and in most cases not at all available commercially. In addition, maximally two reactive silyl groups can be introduced in this way, namely, at the ends of the polymer chain. Products having a higher silicon content, which can be desirable for attaining special effects, e.g., increased spontaneous crosslinking, cannot be produced in this way.
Polyalkenamers which contain silyl groups can be readily prepared by using silyl olefins (German Pat. No. 2,157,405) or silyl cycloolefins (DAS 2,314,543) as the regulators, or as the (co-) monomers during the ring-opening polymerization of cycloolefins; however, here again, general usage is restricted by the lack of commercial availability of the reactants.